1. Field of the Invention
The present invention relates to a variable-magnification image taking device adapted for use in a video camera, a television camera or the like.
2. Related Background Art
In the conventional variable-magnification image taking device, the variation in image magnification is generally achieved by a change in the image angle, by a suitable movement of at least a movable magnification-varying lens group in plural lens groups constituting the phototaking (zoom) lens. In such case, a simple movement of a part of the lens groups results in a movement of the focus position of the object image. In order to compensate such movement in the position of the object image, there is provided a correcting lens group in the phototaking lens, and this correcting lens group is moved by a specified distance so as to maintain the focus position constant.
Therefore, in the conventional variable-magnification image taking device, there are required at least two movable lens groups, namely a movable magnification-varying lens group for zooming operation and a correcting lens group for correcting the focus position. Consequently the configuration of the device becomes inevitably complex, and spaces for the movement of movable lens groups have to be secured therein.
More specifically, in a first example of the zoom lens consisting, in the order from the object side, of a 1st lens group of a positive refractive power, a 2nd lens group of a negative refractive power, a 3rd lens group of a negative refractive power and a 4th lens group of a positive refractive power, in which the 2nd lens group constitutes the movable magnification-varying lens group and the 3rd lens group. constitutes the correcting lens group, said correcting lens group is positioned between the movable magnification-varying lens group and a diaphragm, and the moving space of the correcting lens group, required for the correction of focus position, has to be secured within the zoom lens. As a result, there is required a large distance between the 1st lens group and the diaphragm, and the diameter of the 1st lens group has accordingly to be made large.
Also in a second example of the zoom lens consisting, in order from the object side, of a 1st lens group of a positive refractive power, a 2nd lens group of a negative refractive power, a 3rd lens group of a positive refractive power and a 4th lens group of a positive refractive power, in which the 2nd lens group constitutes the movable magnification-varying lens group and the 3rd lens group constitutes the correcting lens group, the corrective movement thereof is done along a trajectory concave to the object side when the light beam emerging therefrom becomes nearly parallel.
Such corrective movement hinders the zooming operation from the wide-angle position to the middle position. For this reason, there is required a large amount of movement of the 2nd lens group, in order to obtain a predetermined focal length. As a result, there is required a large gap between the 1st and 2nd lens groups, and the diameter of the 1st lens group becomes inevitably large.
Also in a third example of the zoom lens consisting, in the order from the object side, of a 1st lens group of a positive refractive power, a 2nd lens group of a negative refractive power, a 3rd lens group of a positive refractive power and a 4th lens group of a positive refractive power, in which the 2nd lens group constitutes the movable magnification-varying lens group and the 4th lens group constitutes the correcting lens group, the focus point is displaced if an auxiliary equipment such as extender is inserted between the 3rd and 4th lens groups. Stated differently, the focus position varies depending on the insertion of an auxiliary equipment such as extender.
Finally, in a fourth example of the zoom lens consisting, in the order from the object side, of a 1st lens group of a negative refractive power, and a 2nd lens group of a positive refractive power, in which the 2nd lens group constitutes the movable magnification-varying lens group and the 1st lens group constitutes the correcting lens group, the latter tends to become large and heavy, particularly in case of a wide-angle zoom lens. For this reason, the moving mechanism for the correcting lens group has to bear a heavy load.
In recent years, there is an increasing demand for a zoom lens which has a large aperture and a wide image angle, and is of a compact size, a light weight and a high performance, combined with a large zooming ratio. Compactization and reduction in weight of a zoom lens, or a higher performance thereof with same size and weight, can generally be achieved by a decrease in number of the constituting lenses or by an increase in the refractive power of each lens group. As a result, the corrections of various aberrations have to be sacrificed, and the imaging performance becomes inevitably lower.